The present invention relates to a combined polarisation and mode selection technique in a laser and more particularly, though not exclusively, to a gas waveguide laser in which a dielectric surface of the waveguide has a localised metal region located therein. The position and size of the metal region can be varied to provide polarisation and mode selection.
Gas waveguide lasers are the typical choice for many industrial laser processing systems. The laser comprises: first and second surfaces separated to form a waveguide on a first axis between the surfaces; a discharge region contained within at least a portion of the waveguide; and, a resonant cavity having a beam propagation axis orthogonal to the first axis. When only two surfaces are present waveguiding occurs in one dimension on the first axis and the surfaces have a width adequate to allow free-space propagation in the axis orthogonal to the first and propagation axes. This is referred to as a planar waveguide and forms a slab laser. Third and fourth surfaces may be present on a second axis perpendicular to both the first axis and the propagation axis. The distance between the first and second surfaces is typically 1 mm to 3 mm for waveguiding. If the distance between the third and fourth surfaces is greater than 10 times that of the distance between the first and second then this is a planar waveguide. When the third and fourth surfaces are 1 mm to 3 mm apart waveguiding of the light occurs along both dimensions. This is referred to as a channel waveguide.
For a typical planar waveguide the first and second surfaces are metal surfaces such as aluminium. Where third and fourth surfaces are present these will be a dielectric such as alumina. Alternatively, all surfaces may be dielectric guiding surfaces. This may be by application of dielectric on a metallic substrate for the first and second surfaces. In channel waveguides, typical arrangements are for all dielectric surfaces or first and second metallic with third and fourth dielectric. Equally dielectric guiding layers may be formed on metallic substrates. In the prior art each surface over which a discharge is created for waveguiding is typically either entirely metallic or entirely dielectric.
It is known that there exist different mode and polarisation properties for light guided by metallic surfaces as compared to those guided by dielectric surfaces. Alumina guiding surfaces provide better mode selection properties to aluminium surfaces. However, for polarisation selection metallic waveguide surfaces are far superior to dielectric waveguide surfaces.
Along with a low-order mode, a polarisation that is both stable and linear is a necessity for many industrial laser processing systems. In the cutting of thick (>5 mm) steels laser absorption into the side walls of the cut as it penetrates deeper into the material becomes increasingly significant. Since the cut angle is near Brewster's angle the absorption is highly polarisation dependent. To prevent an inconsistency in cut edge quality and machining speeds along orthogonal cut directions, the linear polarisation output of the laser is transformed to circular polarisation using a simple quarter waveplate. If the polarisation from the laser was not linear and stable, the transformed polarisation would be inconsistent and the cut quality and depth would be highly variable. During laser processing of reflective media, particularly metals, a large fraction of the laser light is reflected. This back reflected light can be directed into the laser causing significant variations in the laser output and, in the most extreme cases, damage to optical components within the laser resonator. A linearly polarised output from the laser can be prevented from re-entering after reflection from the work piece by a back reflection device that rotates the return polarisation and absorbs polarisation along a direction perpendicular to the emission exiting the laser. If the polarisation was not linear or stable, the back reflection device would be damaged thereby no longer preventing the back reflected light re-entering the laser. Variable polarisation can also pose challenges when installing the laser. Many beam splitters have polarisation dependent reflectivity. Therefore, installations using a single laser source for multiple beams will have unwanted variation in power along each beam path. Finally, optical modulators for deflecting the beam and varying the temporal profile, such as acousto-optical modulators, have strong polarisation dependence preventing effective operation with variable polarisation lasers.
A standard technique for polarising a laser is to place a Brewster plate into the laser cavity. This introduces significantly higher loss for s-polarised radiation compared with p-polarised radiation. However, this is unsuitable for many waveguide lasers with a gaseous active medium. Typically, the resonator mirrors are placed close to the end of the waveguide, the near-case 1 position, to ensure efficient coupling of the light into the waveguide in a compact device. Therefore, there is insufficient space to fit a Brewster plate. Furthermore, in slab lasers the electrode width is typically >25 mm. Since the Brewster plate would need to extend over the entire width of the beam the component would be large and prohibitively expensive.
In the prior art mode selection for metallic electrodes is provided by spacing the resonator mirrors away from the waveguide exit to introduce increased coupling losses from the resonator mirrors to the waveguide for higher order waveguides. Although effective this approach results in clear constraints on resonator design that can limit the available options when optimising waveguide lasers.